This invention relates to a method for insulating slots in rotors of electrical rotating machines, especially cage type induction motors.
In a prior art cage type induction motor, rotor bars are formed in slots of a rotor by casting, and the slots are insulated to prevent the rotor bars from being short-circuited to cause stray-load loss or to produce stray torque during the operation of the motor.
In insulating the slots, a laminated rotor core assembly formed of a fixed number of laminated rotor punchings is immersed first in a solvent such as acetone to remove punching oil used in punching rotor blanks, and then in a heat-resisting insulating liquid paint or coating. After the laminated rotor core assembly is taken out of the paint or coating, excessive paint in the slots is removed by blowing compressed air into the slots. Thereafter, the assembly is kept in a drying furnace at a predetermined temperature for a given period of time to bake the insulating paint on the inner surfaces of the slots.
After insulating the slot, a cast rotor is manufactured by die-casting aluminium or similar material.
However, the aforementioned method is subject to the following drawbacks.
(1) The solvent used for the removal of the punching oil penetrates into the gaps between the laminated rotor punchings. Accordingly, much solvent is consumed, and in addition, the removal of the solvent requires an extra step of heating it or letting it stand for several hours. From the point of view of environmental sanitation, moreover, an off-line system is essential to the removal of the solvent. Thus, the number of manufacturing processes and cost are increased.
(2) When the rotor core assembly is immersed in the insulating paint, the paint is excessively attached to the shaft hole and the outer peripheral surface of the assembly. Removal of the excessive paint requires a laborious process.
(3) In the immersion process, the insulating paint enters the gaps between the laminated rotor plates by capillarity or a capillary action, leading to much paint consumption. This is not ecomonical. Further, the capillarity will lower the space factor of the rotor core assembly and increase the magnetic resistance of the rotor. Therefore, the exciting current of the rotor is increased so that the power factor of the motor is reduced. Moreover, in the die casting process after the baking process, an axial die fastening pressure is applied to the rotor core assembly to cause cracks or breakage at those portions of the insulating coating film which cover the edge portions of the slots of the rotor punchings. Thus, the insulation between the rotor core assembly and the rotor bars is lowered.
(4) The excessive insulating paint in the slots is removed by allowing it to stand for a time or by blowing away by the use of compressed air. According to the former method, however, the paint is collected at the lower end portions of the slots of the rotor core assembly by surface tension, so that nonconductivity may occur during die casting and impair the electric characteristics of the rotor. According to the latter method, the thickness of the coating films on the inlet portions of the inner walls of the slots where the air is led becomes thinner. Thus, the coating films may be peeled off that inlet portion to lower the insulation effect.
(5) In the conventional art, the coating is dried and hardened by an indirect heating method using ambient air in an electric furnace or the like after insulation treatment. According to this method, however, unduly great thermal energy is consumed, since heat is applied not only to the inner walls of the slots of the rotor punchings, but also to other portions of the rotor punchings which require no heating at all. Moreover, this method requires an additional step and more time for cooling the unnecessarily heated portions.
(6) In the baking process after the insulating treatment, the temperature of the rotor core assembly must be suddenly increased for a very short period of time if the baking temperature of the insulating paint is high, or if the paint is water-soluble. As a result, bubbles will be produced in the insulating paint to lower its electric insulating capability.
These drawbacks would lead to variations in the quality of rotors, and constitute a serious obstacle to the mass production of the rotors.